Cover pictures

Hello My site has become slow, so I'm optimizing it. It's about the service area. All my photo galleries are listed in the service area. I have now set the Select Content Type to Page. This has the advantage that the galleries are sorted according to the creation date. If I want to rearrange this, I can only quickly change the date of the page. But now I have the following problem. Then I choose the featured image on the page. This is displayed at the top of the page when I open it, but at the same time as a contribution image on the service page. Now, if I want to optimize my site, the image for the post on the service site only needs to have a little resolution. But the title of the page, when I open it, is a high-resolution image. How can I do that now? Here as an example: https://vah.ch/#service-section https://vah.ch/galerien/alpen-rundflug/ I have now taken a picture with a small resolution, but then the title picture on the opened page is blurred because the resolution is too low. Many thanks for the help. regards Heinz